Hearing impairment is the most common sensory defect in humans. To faithfully transmit information from hair cells to the brain, developing spira ganglion neurons (SGNs) form different types of specialized synapses. Understanding how these auditory synapses develop may lead to the development of new therapies and broaden opportunities for patient treatment. We have identified the transcription factor MafB as a potential master regulator of auditory synapse development based on its expression pattern, functions in other developing systems, and ability to control the expression of known synaptic molecules. To investigate the function of MafB in auditory synapse development, we generated a floxed allele of MafB and specifically disrupted MafB protein in SGNs. MafB conditional knock-outs (MafBCKO) are viable and exhibit no obvious behavioral abnormalities. Analysis of auditory function in MafBCKO mice by auditory brainstem responses shows that the mutants can still detect sound, consistent with preserved hair cell function. In contrast, the neural response was significantly decreased and delayed relative to controls, suggesting that MafB is required for proper SGN activity. To further dissect a role for MafB, we created a strain of mice (MafBOE) that overexpress MafB upon Cre-mediated recombination. Immunostaining for the ribbon synapse marker RIBEYE/CtBP2 revealed that the number of synaptic ribbons is reduced in MafBCKO and conversely, is increased in MafBOE, suggesting that MafB is required for normal formation of synapses between hair cells and SGNs in the cochlea. These effects may depend in part on the neuronal chemokine CCL21, which activates microglia in other regions of the nervous system. In MafBCKO SGNs, CCL21 expression is dramatically reduced. We will use MafB and CCL21 mutant mouse models to further define the cellular and molecular functions of MafB during auditory synapse development. To achieve these goals, we propose the following experiments. First, we will perform detailed histological and physiological analyses of the MafBCKO and MafBOE mice to evaluate the morphologies of ribbon synapses and the function of SGNs. Second, we will investigate whether MafB works through CCL21 to control microglia and promote synapse maturation. Third, we will use RNA-seq to conduct differential expression analysis and identify MafB downstream genes. Identification of downstream targets will help us understand how MafB coordinates the networks of genes that underlie auditory neuron maturation and synaptogenesis. Together, these studies will provide the opportunity to elucidate the molecular mechanisms of auditory synaptogenesis and may identify novel deafness susceptibility genes.